Multiple myeloma (MM) is a plasma cell cancer characterized by bone marrow clonal plasmacytosis, monoclonal immunoglobulin expression in the serum and/or urine, lytic bone lesions, hypercalcemia, anemia, and renal failure. MM patients initially respond to therapy, but relapse with drug-resistant disease. Therefore, early detection and effective monitoring are critical for management of MM patients. Current clinical assays focus on detection and quantification of the monoclonal immunoglobulin (or M-protein) secreted by the tumor cells, which is essential for diagnosis and monitoring of patients with MM and other plasma cell dyscrasias (PCD). The Durie and Salmon staging system and the International Staging System (ISS) are based on the correlation between the expression of the monoclonal immunoglobulin and the disease burden. The International Myeloma Working Group guidelines also describe the assessment of treatment outcomes based on the changes in expression of the M-protein. Patient monitoring strategies present significant challenges, particularly in the diagnosis of premalignant monoclonal gammopathy of undetermined significance (MGUS), prediction of progression from MGUS to MM, assessment of response to therapy, and detection of relapse.
Evaluation of disease is accomplished by serial measurements of the M-protein in serum and urine using a variety of techniques (Berth M, et al Clin Chem 1999, 45, 309-10; French MAH, et al Clin Exp Immunol 1984, 56, 473-5; Haraldsson A, et al Ann Clin Biochem 1991, 28, 461-66; Chen K, et al Nephrology 2005, 10, 594-596; Buckley R, et al J Clin Invest 1975, 55, 157-65; Hunder G, et al Arthritis Rheum 1974, 17, 955-63). Typically, initial measurements are made using serum protein electrophoresis (SPEP), which is limited in sensitivity to approximately 0.1 gram per deciliter (g/dl) (O'Connell T X, et al Am Fam Phys 2005, 71, 105-112). The monoclonal immunoglobulin produced in high concentration by MM cells can be visualized as a narrow, discrete, dark band usually in the γ region of the gel or electropherogram. SPEP densitometry and total serum protein concentration are used to estimate the amount of the immunoglobulin secreted by the tumor. Patients can be further characterized using immunofixation electrophoresis (IFE). IFE screens test for immunoglobulin G, A, and M heavy chains, as well as kappa (κ) and lambda (λ) light chains. Immunoglobulin D and E myelomas are rare; when suspected, IFE is repeated to detect IgD or IgE. The combination of SPEP and IFE establishes an estimated level in the serum and type of the immunoglobulin that is secreted by the tumor. These traditionally gel-based techniques have recently been replaced by capillary array instruments (Jolliff C R, et al Electrophoresis 1997, 18, 1781-4). For immunoglobulin heavy chains with high expression, SPEP is the current clinical standard for detecting tumor burden, because the disease-specific immunoglobulin is directly monitored.
However, several factors limit SPEP in monitoring tumor burden in patients (Morita K, et al Clin Lab 2004, 50, 415-18; Schreiber W E, et al Am J Clin Pathol 1992, 97, 610-13). Therefore, quantification of the involved immunoglobulin by nephelometry is also used to monitor tumor burden (Clark R, et al Electrophoresis 1998, 19, 2479-84), and it has particular value for immunoglobulins with lower abundances in serum (e.g. IgD and IgE), particularly because the background expression of these immunoglobulins is low. Serum free light chain assays (SFLC) are also implemented using nephelometry to provide an expression ratio between the light chains, which supplements other techniques for the detection of light chain only disease (Jagannath S Clin Lymphoma Myeloma 2007, 7, 518-23; Tate J R, et al Clin Chim Acta 2007, 376, 30-6; Pika T, et al Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub 2008, 152, 61-4). Antibody-based methods for protein quantification are also influenced by the complexity of the immunoglobulin system of the biologically derived antiserum and variation in its reactivity, as well as changes in the levels of proteins in the standard reagents (typically pooled serum) (Reimer C B, et al Clin Chem 1976, 22, 577-82). The presence of immunological subclasses (i.e. IgG1-4 and IgA1-2) also adds to the complexity of the analysis (Reimer C B, et al Clin Chem 1976, 22, 577-82).
Minimal residual disease (MRD) is the name given to small numbers of leukaemic cells that remain in the patient during treatment, or after treatment when the patient is in remission. It is the major cause of relapse in cancer and leukaemia. However, conventional SPEP is not sufficiently effective for detecting MRD.
A quantitative proteomic assay personalized for an individual patient and useful for direct measurement of the disease-specific immunoglobulin would significantly increase the sensitivity of detection over SPEP and allow for detection of MRD so treatment can be altered.